1. Field of the Invention
The present invention relates to a photodetector with a built-in circuit, in which a photodiode for converting incident light into an electric signal and an integrated circuit for processing the converted signal are provided on the same silicon substrate, and to a method for producing such a photodetector.
2. Description of the Related Art
Photodetectors with a built-in circuit are used in a wide range of applications such as, particularly, optical pickups and optical space transmission. In optical pickups, photodetectors with a built-in circuit are used to detect a focus error signal for adjusting a focal position of semiconductor laser light on a disk or a radial error signal for adjusting a focal position of semiconductor laser light to a pit in the disk (i.e., tracking). In recent years, there has been an increasing demand for an improvement in speed and sensitivity of photodetectors with a built-in circuit.
FIG. 16 shows a conventional photodetector with a built-in circuit disclosed in Japanese Laid-Open Publication No. 10-107243. A photodetector with a built-in circuit 600 includes a P-type semiconductor substrate 603, a photodiode 601, and an integrated circuit 602. The photodiode 601 includes a P-type buried isolation diffusion layer 102A, an N-type buried diffusion layer 103A, an N-type epitaxial layer 104, a P-type isolation diffusion layer 105A, a P-type diffusion layer 107, a silicon thermal oxide film 111, and a silicon nitride film 112. The integrated circuit 602 includes a P-type buried isolation diffusion layer 102B, an N-type buried diffusion layer 103B, the N-type epitaxial layer 104, a P-type isolation diffusion layer 105B, a collector compensation diffusion layer 106, an NPN transistor external base diffusion layer 109, an NPN transistor internal base diffusion layer 108, an NPN transistor emitter diffusion layer 110, the silicon thermal oxide film 111, the silicon nitride film 112, a first-layer conductor 113, an interlayer insulation film 114, a second-layer conductor 115, and a silicon nitride film 116.
In order to speed up the photodiode 601, it is necessary to reduce a diffusion current components and a CR time constant component both of which have slow response. The diffusion current component is reduced by providing the N-type buried diffusion layer 103A and the P-type diffusion layer 107 in the neighborhood of an isolation portion (i.e., the P-type buried isolation diffusion layer 102A and the P-type isolation diffusion layer 105A, respectively). The CR time constant component is reduced by decreasing the capacitance CPD of the photodiode 601. Therefore, the N-type buried diffusion layer 103A and the P-type diffusion layer 107 are each designed to have as small a size as possible but at which practical use is still allowable. The P-type diffusion layer 107 is provided in a region which is irradiated with a laser beam reflected from a disk (not shown) when reading a signal from the disk.
An antireflection film is provided on a light receiving surface of the photodiode 601 so as to improve sensitivity of the photodiode 601. The silicon thermal oxide film 111 and the silicon nitride film 112 form a laminated layer which serves as the antireflection film.
The silicon thermal oxide film 111 needs to be provided on the light receiving surface of the photodiode 601. This is because a junction surface of the P-type diffusion layer 107 and the N-type epitaxial layer 104 reaches the light receiving surface of the photodiode 601, so that there occurs a leakage current on the light receiving surface.
The silicon thermal oxide film 111 and the silicon nitride film 112 are provided in such a manner as to have a low reflectance with respect to laser wavelengths used for CD-ROMs and DVD-ROMs (i.e., 780 nm and 650 nm, respectively).
In the integrated circuit 602, device isolation is achieved by diffusion isolation. The NPN transistor external base diffusion layer 109 and the NPN transistor internal base diffusion layer 108 are formed by implantation of boron (B+) ions. The NPN transistor emitter diffusion layer 110 is formed by implantation of arsenic (As+) ions. The thus-constructed NPN transistor has a maximum frequency (fTmax) of 3 GHz, and a response of as high as 60 MHz for a photodetector with a built-in circuit.
However, there is a demand for even higher-speed photodetectors with a built-in circuit. To meet the demand, production methods of Poly-Si emitters, Poly-Si bases, emitters having a double polysilicon structure, and the like have been developed.
Among transistors having such structures, heterojunction bipolar transistors (hereinafter referred to as xe2x80x9cHBTxe2x80x9d) which employ a heterojunction such as Si/SiGe have drawn attention in recent years. In the HBT, an emitter-base junction is formed between two substances having different bandgaps (e.g., Si and SiGe). The HBT allows a barrier height against holes injected from a base into an emitter to be higher than that against electrons injected from the emitter into the base, so that carrier density in the base region can be increased without decreasing the efficiency of injection from the base into the emitter. Accordingly, the HBT allows base resistance, which is increased due to miniaturization, to be decreased, thereby speeding up the transistor.
In an attempt to obtain a high-speed photodetector with a built-in circuit, the photodiode 601 shown in FIG. 16 and the integrated circuit 602 sped up by employing the HBT may be provided on the same P-type semiconductor substrate 603. In this case, however, there arises the following problem.
For the photodiode 601 of FIG. 16 in which a PN junction of the P-type diffusion layer 107 and the N-type epitaxial layer 104 are formed, if a film deposited by CVD or the like is provided as an antireflection film on the light receiving surface of the photodiode 601, leakage current is increased on the surface of the photodiode 601. To avoid this, the silicon thermal oxide film 111 as an antireflection film is required.
As explained above, diffusion layers 108 and 109 in Prior Art FIG. 16 are formed by implantation of boron (B+) ions (they are not made of SiGe). However, in order to explain certain embodiments of the instant invention, if SiGe layers were to be provided as the NPN transistor external base diffusion layer 109 and the NPN transistor internal base diffusion layer 108 of the integrated circuit 602 (HBT), such layers may have a strain caused by lattice mismatch due to a difference in a lattice constant between Si and Ge. Accordingly, when the SiGe layers are formed at a high temperature, dislocation occurs at an interface between the Si layer and the SiGe layer, thereby increasing a generation recombination current.
In the case where the silicon thermal oxide film 111, which serves as the antireflection layer, is formed after the formation of the SiGe layer, strain energy held in the strain caused by the lattice mismatch of the SiGe layer is released. This leads to lattice relaxation which triggers the occurrence of dislocation, so that junction leakage characteristics of the NPN transistor of the integrated circuit 602 are deteriorated. Moreover, the composition of the SiGe layer is changed such that the resultant SiGe layer does not have desired characteristics (e.g., bandgap).
After the NPN transistor external base diffusion layer 109 and the NPN transistor internal base diffusion layer 108 are formed, the NPN transistor emitter diffusion layer 110, the first-layer conductor 113, and the second-layer conductor 115 are formed. Typically, dry etching is used to etch Poly-Si when forming the NPN transistor emitter diffusion layer 110. Dry etching is also used to etch AlSi which is usually used as a material for the first-layer conductor 113 and the second-layer conductor 115. The silicon nitride film 116 is etched with a gas. For example, the silicon nitride film 116 is made thinner at a rate of 4-5 nm per minute by the gas used in etching AlSi. As a result, the thickness of the silicon nitride film 116 departs from an optimum thickness for a low reflectance.
According to the present invention, there is provided a photodetector with a built-in circuit which includes a semiconductor substrate, an integrated circuit provided on the semiconductor substrate, and a photodiode provided on the semiconductor substrate. The integrated circuit includes a SiGe layer provided on at least a portion of the integrated circuit. Thus, the above described objective is achieved.
The photodiode may include a homojunction of Si.
The photodiode may include a plurality of split photodiodes.
The photodiode may include an antireflection film provided on the semiconductor substrate and the antireflection film may include a silicon thermal oxide film.
The antireflection film may further include a silicon nitride film.
A thickness of the silicon thermal oxide film may be between about 10 nm or more and about 40 nm or less.
The semiconductor substrate may include a first conductivity-type semiconductor having a high resistivity.
The semiconductor substrate may include a first conductivity-type semiconductor substrate having a low resistivity and a first conductivity-type epitaxial layer provided on the first conductivity-type semiconductor substrate which has a resistivity higher than that of at least the first conductivity-type semiconductor substrate.
The semiconductor substrate may include a first conductivity-type semiconductor substrate having a low resistivity, a first conductivity-type semiconductor layer provided on the first conductivity-type semiconductor substrate which has a resistivity lower than that of at least the first conductivity-type semiconductor substrate, and a first conductivity-type epitaxial layer provided on the first conductivity-type semiconductor layer which has a resistivity higher than that of at least the first conductivity-type semiconductor substrate.
According to the present invention, there is provided a method for producing a photodetector with a built-in circuit. The photodetector with a built-in circuit includes a semiconductor substrate, an integrated circuit formed on the semiconductor substrate, and a photodiode provided on the semiconductor substrate. The integrated circuit includes a SiGe layer formed on at least a portion of the integrated circuit. The method includes the steps of: a) forming the photodiode on the semiconductor substrate; and b) forming the SiGe layer after the photodiode is formed. Thus, the above described objective is achieved.
The photodiode may include an antireflection layer provided on the semiconductor substrate. Step a) may include the step of c) forming the antireflection layer on the semiconductor substrate. Step c) may include the step of forming a silicon thermal oxide film on the semiconductor substrate. Step b) may include the steps of: etching the silicon thermal oxide film formed on step c); and forming the SiGe layer by low temperature MBE.
Step c) may include the steps of forming a silicon nitride film on the silicon thermal oxide film and simultaneously forming a silicon nitride film used as a silicon nitride film capacitance portion in the integrated circuit.
Step c) may include the step of forming a silicon oxide film on the silicon nitride film so as to protect the silicon nitride film.
Step c) may include the step of d) etching the silicon oxide film after all dry-etching processes are completed.
The method for producing a photodetector with a built-in circuit may further includes the step of (e) forming a cover insulation film to pattern the cover insulation film by dry-etching. Step d) may be executed after the execution of step e).
Step d) may include the step of etching the silicon oxide film while the cover insulation film functions as a protection film.
According to an aspect of the invention, the SiGe layer of an HBT is formed after the formation of a silicon thermal oxide film. Therefore, a high temperature caused by thermal treatment for forming the silicon thermal oxide film substantially does not affect the SiGe layer. As a result, the SiGe layer can be formed to have desired characteristics without undergoing a change in the composition thereof. Furthermore, since a high temperature caused by thermal treatment substantially does not affect the SiGe layer, strain energy is not released. As a result, lattice relaxation does not trigger the occurrence of dislocation, so that junction leakage characteristics of the transistor are not deteriorated. Accordingly, it is possible to obtain a high-speed and high-sensitive photodetector with a built-in circuit which includes a high-speed integrated circuit employing an HBT and a photodiode having a high sensitivity (a low reflectance).
According to another aspect of the present invention, an antireflection film is formed of a silicon thermal oxide film and a silicon nitride film formed thereon. Therefore, a photodiode having a lower reflectance and higher sensitivity can be obtained. The silicon nitride film can be formed simultaneously with a silicon nitride film used as a silicon nitride film capacitance portion without increasing the production cost.
According to still another aspect of the present invention, the thickness of the silicon thermal oxide film used as an antireflection film is between about 10 nm or more and about 40 nm or less. Therefore, the reflectance can be kept low with respect to laser beam wavelengths used for CD-ROMs and DVD-ROMs.
According to still another aspect of the invention, when the silicon nitride film is formed as an antireflection film, a protective silicon oxide film for protecting the silicon nitride film is formed thereon so as to prevent the silicon nitride film from being made thinner during dry-etching steps. Thus, increase in reflectance caused by a change from an optimal thickness of the silicon nitride film is prevented, so that the sensitivity of the photodiode is maintained.
According to still another aspect of the present invention, a protective silicon oxide film for protecting the silicon nitride film is removed by wet-etching after all the dry etching steps are performed, i.e. after the cover insulation film is patterned by the final dry etching step. Thus, the silicon nitride film as an antireflection film can be protected from all the dry etching steps.
According to still another aspect of the present invention, the cover insulation film is used as a mask when wet-etching is performed. Therefore, a photolithography step for etching the protective silicon oxide film is not required and the cost of photolithography can be kept from increasing.
According to still another aspect of the present invention, the semiconductor substrate includes a first conductivity-type semiconductor having a high resistivity. By using such a substrate, it is possible to further speed up the photodiode.
According to still another aspect of the present invention, the semiconductor substrate includes a first conductivity-type semiconductor substrate having a low resistivity, and a first conductivity-type epitaxial layer provided on the first conductivity-type semiconductor substrate which has a resistivity higher than that of at least the first conductivity-type semiconductor substrate. By using such a substrate, the capacitance and the series resistance of the photodiode both can be reduced so that a CR time constant component can be reduced. Thus, it is possible to further speed up the photodiode.
According to still another aspect of the present invention, the semiconductor substrate includes a first conductivity-type semiconductor substrate having a low resistivity, a first conductivity-type semiconductor layer provided on the first conductivity-type semiconductor substrate which has a resistivity lower than that of at least the first conductivity-type semiconductor substrate, and a first conductivity-type epitaxial layer provided on the first conductivity-type semiconductor layer which has a resistivity higher than that of at least the first conductivity-type semiconductor substrate. By using such a substrate, the capacitance and the series resistance of the photodiode both can be reduced. Moreover, photocarriers generated at a deep position are accelerated by internal field caused by the first conductivity-type semiconductor layer having a resistivity lower than that of the first conductivity-type semiconductor substrate, thereby further speeding up the photodiode.
According to still another aspect of the present invention, a high-speed integrated circuit which is sped up by employing an HBT and a high-speed and high-sensitive split photodiode can be formed on the same substrate by forming the antireflection film including the silicon thermal oxide film which is necessary for increasing the speed and sensitivity of the photodiode and the SiGe layer of an HBT. An antireflection film is formed by laminating a silicon thermal oxide film on a silicon nitride film so as to improve sensitivity of the photodiode. In such a structure, the protective silicon oxide film is formed and then wet-etched after the subsequent dry etching step. This prevents the silicon nitride film from being made thinner, thereby maintaining a high sensitivity of the photodiode.
Thus, the invention described herein makes possible the advantages of providing: a photodetector with a built-in circuit in which an integrated circuit sped up by employing an HBT which includes an SiGe layer, and split photodiodes can be provided on the same substrate; and a method for producing such a photodetector.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.